1. Field of the Invention
The present invention relates to a vibration suppressing method for use in a machine tool which performs a machining operation on a workpiece with a tool while rotating the tool or the workpiece, for suppressing chatter vibrations generated during the machining operation, and a vibration suppressing device for executing such a vibration suppressing method.
2. Description of Related ART
An example of hitherto-known vibration suppressing methods for use in a machine tool which performs a machining operation with a rotary shaft (i.e., main spindle) being rotated is disclosed in Japanese Patent Application Laid-Open Publication 2003-340627 In this vibration suppressing method, in order to suppress chatter vibrations which would cause deterioration in the accuracy of a finished surface of the workpiece, destruction of the tool and other detrimental effects, natural frequencies of systems (e.g., a tool, a workpiece, etc.) in which the chatter vibrations occur are determined. Then, the determined values are multiplied by 60 and divided by the number of tool flutes and a predetermined integer to obtain a value assumed to be a stable rotation speed, so that the chatter vibrations which would occur during the machining operation can be suppressed by performing the operation with this stable rotation speed. It is to be understood that the obtained stable rotation speed includes several values such as a first speed obtained with integer “1”, a second speed obtained with integer “2”, and the like.
Many of the generally prevailing machine tools known in the art practically adopt a method of changing gears according to the rotation speeds of the main spindle in order to acquire an adequate motor torque over a broad range of rotation speeds of the main spindle. In cases where higher-speed rotation of the main spindle is required, also, many known machine tools adopt a method of changing a preload of bearings for supporting the main spindle according to the rotation speeds of the main spindle in order to prevent the main spindle from being damaged by heat. These methods of changing the gears or the preload of the bearings would change the dynamic characteristics of the main spindle system according to the rotations speeds of the main spindle.
Referring back to Japanese Patent Application Laid-Open Publication 2003-340627, the stable rotation speeds obtained to suppress chatter vibrations are derived from the “natural frequencies of systems in which the chatter vibrations occur”, as described above. Since the dynamic characteristics of the main spindle system change according to the rotation speeds of the main spindle, some of the obtained stable rotation speeds would possibly become rotation speeds at which the chatter vibrations could not be suppressed due to the change of the dynamic characteristics of the main spindle system when the rotation speed of the main spindle is changed to such a would-be stable rotation speed. As a result, even when the rotation speed of the main spindle is changed to one of the obtained stable rotation speeds, the method possibly fails to prevent the chatter vibrations in some instances.
The present invention has been made in an attempt to eliminate the aforementioned disadvantages, and illustrative, non-limiting embodiments of the present invention overcome the above disadvantages and other disadvantages not described above.